An epoxy resin composition containing an epoxy resin and an epoxy compound (these are sometimes generically referred to as an “epoxy compound”) can be converted to a lightweight cured product having high strength by curing the epoxy resin composition. Therefore, the epoxy resin composition is widely used as materials for constituting products and parts or the like in various fields. In recent years, for example, there is an attempt to use the epoxy resin composition as a material for forming a syntactic foam (e.g., see Patent Literature 1). The syntactic foam is a structure where microballoons (minute hollow spherical bodies) are dispersed in a matrix resin (polymer matrix). Taking advantage of light weight and high strength characteristics, the syntactic foam is used as a buoyant body of a deep ocean research vehicle, or the like.